1. Field of the Invention
The present invention relates to a CRT display device in which a spot burn of CRT is prevented in the television receiver or CRT monitor.
2. Description of the Related Art
There was the conventional technique for preventing a spot residue of CRT as proposed in JP-A-4-2169. That is, with this technique, white peak adjusting means sends an adjusting signal for shifting compulsorily a white peak of video signal to the high brightness side to a white peak adjusting signal input section, if a signal for transfer to the stand-by state is input. Therefore, when transferring to the stand-by state, the CRT becomes luminous at high brightness, so that cumulative charges of the CRT are discharged, preventing the spot residue from occurring.
However, though by the use of the above constitution, the circuit becomes complicated, a technique for preventing the spot residue while avoiding the complication of the circuit was proposed in JP-A-8-317245. That is, in this technique, a brightness signal is led via an image quality adjusting circuit and a contrast adjusting circuit to a pedestal clamp circuit. Also, the brightness adjustment for the brightness signal is made by leading an output of the brightness adjusting circuit to the pedestal clamp circuit. Also, an ABL circuit is provided, with its output being lead to the brightness adjusting circuit, to suppress a rise in the brightness. Also, the image quality adjusting circuit, the contrast adjusting circuit and the brightness adjusting circuit are operated under the control of a microcomputer.
In the above constitution, when the power supply is turned off, the microcomputer issues an instruction for making the brightness maximum to the brightness adjusting circuit. Therefore, since the brightness of a signal sent from the pedestal clamp circuit is increased, a cathode voltage of CRT is controlled to be minimum, so that an anode current of CRT increases. In this state, the microcomputer turns off the power supply for the circuits associated with the CRT. Accordingly, when the power supply is turned off, cumulative charges of the CRT are discharged promptly, preventing the occurrence of spot residue.
However, the brightness adjusting circuit adjusts the brightness of an image displayed on the CRT to the user's taste. Accordingly, when a video signal led to the pedestal clamp circuit represents a dark image like a night view, the brightness of a signal sent from the pedestal clamp circuit may not be increased to a sufficient level, even if an instruction for increasing the brightness to the maximum is issued to the brightness adjusting circuit. In such cases, there is the risk that the discharge is not sufficient to cause the spot residue.
As described in the conventional technique as above cited, “the microcomputer 111 instructs the brightness adjusting circuit 105 to increase the brightness to the maximum, immediately before instructing to turn off the power supply”, the microcomputer instructs the brightness adjusting circuit, but issues no instruction to the ABL circuit. Consequently, the operation of the brightness adjusting circuit is restrained by the operation of the ABL circuit, so that the brightness may not be increased sufficiently. And if the brightness is not increased sufficiently, the discharge is not made sufficiently, whereby there is the risk that the spot residue may occur.